[material-ui][docs] Document callback to access theme in GlobalStyles

[aarongarciah]

Adds a demo on how to access the theme when using GlobalStyles. It's a convenient pattern that is undocumented.

Preview link: https://deploy-preview-42257--material-ui.netlify.app/material-ui/customization/how-to-customize/#3-global-theme-overrides

[mui-bot]

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• docs/data/material/customization/how-to-customize/how-to-customize.md

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